This invention relates to cardiac stimulating devices and particularly to implantable cardiac stimulating devices, including implantable cardiac pacemakers and implantable cardiac defibrillators, as well as implantable cardioverters and cardioverter/defibrillators. More particularly, this invention relates to implantable leads for such cardiac stimulating devices, which incorporate cardiac wall motion sensors that provide signals indicative of cardiac mechanical activity.
Implantable cardiac stimulating devices for providing therapy in response to a variety of pathological cardiac arrhythmias are known. For example, an implantable cardiac stimulating device may be capable of detecting a pathological cardiac arrhythmia, and responding to the detected arrhythmia by providing therapeutic electrical stimulation. Implantable cardiac stimulating devices may be capable of providing "tiered therapy," in which the type of electrical stimulation provided by the device is determined in accordance with the severity of the arrhythmia, with more aggressive therapies being applied in response to more severe arrhythmias. For example, an implantable cardiac stimulating device may respond to a relatively less severe occurrence of tachycardia by delivering antitachycardia pacing pulses of about 25 microjoules to about 30 microjoules in a sequence known to interrupt such an arrhythmia. In response to a relatively more severe occurrence of tachycardia, the implantable cardiac stimulating device may deliver a low energy shock on the order of about 2 joules to about 5 joules, either in combination with, or as an alternative to, antitachycardia pacing pulses. In response to an occurrence of an even more severe arrhythmia, for example, ventricular fibrillation, the implantable cardiac stimulating device may deliver a high energy "defibrillation" shock on the order of about 10 joules to about 40 joules.
Implantable cardiac stimulating devices for providing pacing pulses to cardiac tissue to maintain a heart rate at a physiologically acceptable rate (i.e.--to provide "bradycardia pacing support") are also known. Bradycardia pacing support may be provided by a dedicated pacemaker, or by a device that is also capable of providing other forms of therapy, such as tiered therapy.
Effective delivery of therapy from an implantable cardiac stimulating device depends upon accurate measurement of intrinsic cardiac activity. In the case of an implantable cardiac stimulating device capable of providing tiered therapy, the device must not only be capable of detecting the onset of an arrhythmia, but must also be capable of discriminating among various types of arrhythmias in order to deliver an appropriate form of electrical stimulation therapy. For example, if ventricular fibrillation is incorrectly diagnosed by the device as a relatively less severe arrhythmia, valuable time may be lost if an inappropriate, less aggressive therapy, such as antitachycardia pacing, is applied. If tachycardia is incorrectly diagnosed as ventricular fibrillation, the patient may consciously experience high energy defibrillation shocks, which may be ineffective in terminating the tachycardia, in addition to being extremely uncomfortable.
Measurement of intrinsic cardiac activity is also desirable for implantable cardiac stimulating devices capable of providing bradycardia pacing support. Typically, the delivery of bradycardia pacing pulses from such devices is inhibited by spontaneous, hemodynamically effective, cardiac contractions occurring at a predetermined rate. For example, if the intrinsic heart rate of a patient during a particular time interval is greater than a predetermined threshold rate, delivery of pacing pulses may be inhibited during that time interval. Pacing pulses would be provided when the intrinsic heart rate falls below the threshold rate. Pacing pulse inhibition is desirable because it extends battery life by avoiding delivery of unnecessary stimulation pulses. In order for a device to be capable of inhibiting delivery of pacing pulses, it must be capable of detecting intrinsic cardiac activity.
Many implantable cardiac stimulating devices that detect and discriminate among cardiac arrhythmias monitor heart rate, which is usually accomplished by measuring cardiac electrical activity--i.e., the intercardiac electrogram (IEGM). The IEGM is typically sensed by electrodes that are also used to deliver electrical stimulation therapy to the cardiac tissue. However, under many circumstances, it is difficult to sense the IEGM. For example, the device may not be able to discern the IEGM over noise or other physiological electrical activity, or perhaps even external interference. As a result, an implantable cardiac stimulating device may have difficulty detecting the onset of an arrhythmia. As another illustration, implantable cardiac stimulating devices capable of providing bradycardia pacing support may be inhibited from sensing cardiac electrical activity during a period of time immediately following the delivery of a pacing pulse, due to the presence of a pulse-induced after-potential.
Other known implantable cardiac stimulating devices use hemodynamic signals to detect cardiac arrhythmias. For example, U.S. Pat. No. 4,774,950 to Cohen refers to a system that may detect cardiac arrhythmias by measuring mean pressure at a variety of locations (e.g., mean arterial pressure, mean right ventricle pressure, mean left atrial pressure, mean left ventricle pressure or mean central venous pressure). For a selected mean pressure, a short term current mean pressure is compared to a long term mean baseline pressure, and if they differ by a predetermined valve, the patient may be deemed to be experiencing a cardiac arrythmia. The mean pressure data may also be used in combination with heart rate measurements to detect arrhythmias.
Another example of a device that uses hemodynamics to detect cardiac arrhythmias is described in U.S. Pat. No. 4,967,748 of Cohen. In that patent, blood oxygen level is measured at a particular site in the circulatory system of a patient. A comparison is made between a short term sensed blood oxygen level and a baseline blood oxygen level, and if they differ, the patient may be deemed to be experiencing a cardiac arrhythmia.
Unfortunately, the use of hemodynamic indicators such as mean pressure and blood oxygen level may have certain associated drawbacks. One possible drawback is that hemodynamic indicators may not respond rapidly to the onset of an arrhythmia. Thus, an implantable cardiac stimulating device that relies on such hemodynamic signals to detect cardiac arrhythmias may not deliver therapy as rapidly as desired.
In view of the deficiencies associated the use of the IEGM or certain hemodynamic indicators, it would be desirable to provide an improved sensor for detecting and discriminating among various cardiac arrhythmias, and for determining the intrinsic heart rate of a patient. Ideally, such a sensor would provide a signal that rapidly responds to the onset of an arrhythmia, and is not subject to electrical interference from external sources or from pacemaker-induced after potentials.